The elements of a group show the following important similar characteristics :
Electronic configuration—All elements in a
particular group have similar outer electronic
configuration e.g., all elements of group 1, i.e.,
alkali metals have n${{s}^{1}}$ configuration in their
valency shell. Similarly group 2 elements (alka-
line Earths) have n${{s}^{2}}$ outer configuration and
halogens (group 17) have n${{s}^{2}}$ n${{p}^{5}}$ configuration
(where n is the outermost shell).
(ii) Valency—The valency of an element depends
upon the number of electrons in the outermost
shell. So elements of a group show the same
valency, e.g., elements of group 1 show + 1
valency and group 2 show + valency i.e.,
valency i.e., NaCl, Mg$C{{l}_{2}}$ etc.
(iii) Chemical properties—The chemical properties
of the elements are related to the number
of electrons in the outermost shell of their
atoms. Hence all elements belonging to the
same group show similar chemical properties. But the degree of reactivity varies gradually from top to bottom in a group. For example, in group I all the elements are highly reactive
metals but the degree of reactivity increases
from Li to Cs. Similarly elements of group 17,
i.e., halogens : F, Cl, Br, I are all non-metals
and their reactivity goes on decreasing from
top to bottom.